In a radio communication system, radio spectrum resources may be divided into multiple sub-bands and the multiple sub-bands may be multiplexed. According to a division mode of the spectrum resources, multiplexing technologies may include time-domain multiplexing, frequency-domain multiplexing, space-domain multiplexing, code-domain multiplexing and amplitude-domain multiplexing, etc. A multiplexed transmission may include: orthogonal multiplexed transmission and non-orthogonal multiplexed transmission.
The orthogonal multiplexed transmission allows that signals may be transmitted on the multiple sub-bands in a non-interference mode. A receiver may demodulate a signal carried by each sub-band through independent processing, which means that a complexity degree of the receiver is low. Therefore, the orthogonal multiplexing, such as the time-domain orthogonal multiplexing and the frequency-domain orthogonal multiplexing is widely used in various standards of a communication system. If the multiple sub-bands multiplexed in the communication system are allocated to different users, different multiple access technologies, such as a Time Division Multiple Access (TDMA) technology, a Frequency Division Multiple Access (FDMA) technology and a Code Division Multiple Access (CDMA) technology may be used.
In a Long Term Evolution (LTE) system corresponding to an Evolved Universal Terrestrial Radio Access (E-UTRA) protocol made by the 3GPP (3rd Generation Partnership Project), the Orthogonal Frequency Division Multiplexing (OFDM) technology and Orthogonal Frequency Division Multiple Access (OFDMA) technology are used in a Downlink Link (DL). In the downlink link, since there is an orthogonal relationship between the sub-bands, signals transmitted to multiple user equipment (UEs) from the eNB may be multiplexed in the frequency domain. Thus, in the LTE system, each UE may only need to demodulate a signal on a sub-band allocated to the UE and need not to pay attention to the interference from other UEs. The low complexity degree of the receiver and high frequency utilization rate may enable efficient use of communication resources in the LTE system.
However, according to the research of an information theory, the maximum channel capacity cannot be obtained with an orthogonal multiple access scheme in a fading channel. Signals of multiple users may be superimposed together in an amplitude domain with a non-orthogonal multiple access scheme. Therefore, a multi-user gain may be obtained among users with relatively large channel gain differences and total throughput of the communication system may be increased. As spectrum resources of the communication system are increasingly scarce and demand for radio communication services rapidly grows, the non-orthogonal transmission technology, in which the signals of multiple users are superimposed in the amplitude domain, is introduced into the communication system, which may provide the future communication system with higher throughout. Hereinafter, transmission that uses an orthogonal multiplexing (access) scheme is called orthogonal transmission, and transmission that uses a non-orthogonal multiplexing (access) scheme is called non-orthogonal transmission. Signals of multiple users (or UEs) superimposed together in the amplitude domain in the non-orthogonal transmission are called multi-layer signals.
In the non-orthogonal transmission, the eNB may superimpose multi-layer signal amplitudes together and send the superimposed multi-layer signals to one or multiple UEs. A UE may take signals of other user layers as noises and demodulate a signal of the UE's layer. Other UEs may need to demodulate the signals of other layers superimposed on the UE's layer, delete the demodulated signals of other layers from a received signal with the well-known Successive Interference Cancellation (SIC) receiver and demodulate the signal of the UE's layer.
FIG. 1 illustrates a wireless communication system, in which DL signals of UEs are superimposed. DL signals 11, 13 transmitted from an eNB 111 to UEs 101, 103 may be superimposed. The DL signal 11 transmitted to the UE A 101 is represented by signal A 11, and the DL signal 13 to the UE B 103 is represented by signal B 13. The signals A and B 11 and 13 are examples of the multi-layer signals.
FIG. 2 illustrates how to receive signals in the wireless communication system of FIG. 1.
Referring to FIG. 2, UE A 101 receives and demodulates the signal A 11 in operation 201. UE B 103 receives and demodulates the signal A 11 in operation 203, re-establishes the signals A 11 in operation 205, deletes the signal A 11 from a signal received though an SIC receiver in operation 207, and demodulates the signal B 13 in operation 209.
However, in order to make the non-orthogonal transmission sharing the amplitude domain effectively apply to a communication system, a serial of technical challenges may need to be overcome. Otherwise, advantages of the non-orthogonal transmission can only stop at the theoretical analysis. Hereinafter, the non-orthogonal transmission is to be understood as sharing the amplitude domain. Some technology details for implementing the non-orthogonal transmission may be described hereinafter.
1) If an eNB schedules non-orthogonal transmission, one or some UEs may first need to demodulate data of other users. Otherwise, excessive interference may result in that the UEs cannot demodulate signals belonging to them. Therefore, the eNB may need to inform the UE how to demodulate signals of the other users. Since the eNB may switch between non-orthogonal transmission scheduling and conventional orthogonal transmission scheduling, the UE may need to be dynamically informed of the non-orthogonal scheduling
2) If a UE demodulates a multi-layer signal, the UE may need to know amplitude information and phase information of a channel on which the multi-layer signal is transmitted. Although a conventional reference signal may provide the phase information of the channel, the UE cannot accurately estimate amplitude information of each signal. Thus, with design of the reference signal in the orthogonal transmission, the UE cannot accurately demodulate the multi-layer signal in the non-orthogonal transmission.
3) In the orthogonal transmission, the UE may assume that the signal from an eNB may only include the signal belonging to the UE. Then, the UE may calculate channel state information according to measured background noises and interference from the other cells and feed back the channel state information to the eNB. However, non-orthogonal transmission in which signals of multiple UEs are superimposed in the amplitude domain means that a receiving signal of the UE is affected by extra interference and the interference only occurs if the receiving signal arrives, resulting in that it is very difficult to predict the interference and feed back a piece of accurate channel state information. It is difficult for the conventional design and technology to effectively support non-orthogonal transmission.
For instance, it may be assumed that an eNB may allocate half power to UE A and adopt Quadrature Phase Shift Keying (QPSK) modulation. Furthermore, the eNB may allocate the other half of the power to a UE B and adopt 16 Quadrature Amplitude Modulation (QAM). Moreover, it is assumed that the eNB may send a reference signal with full power. In this case, the UE B first needs to demodulate data of the UE A and delete the UE A's interference from a received signal. The UE B obtains channel information according to the reference signal, where is the amplitude information of the channel, is the phase information of the channel. Based on the phase information of the channel, the UE B may demodulate the signal of the UE A and delete the UE A's interference from the received signal. However, since the UE B cannot obtain the amplitude information of the 16QAM in the non-orthogonal transmission and cannot demodulate the UE A's signal.
For another example, it may be assumed that the UE A recommends using the 16QAM transmission mode in channel state feedback according to the background noises and an interference situation of an adjacent cell. However, if the eNB superimposes a low-power signal on a 16QAM signal, the interference received by the UE A may likely be the interference from the superimposed low-power signal, other than the interference from the background noises or the adjacent cell. Therefore, in this case, the UE A may fail to perform the demodulation due to extra interference.
Therefore, a need exists for a scheme for effectively applying the non-orthogonal transmission to the DL link in the radio communication system.